Radio frequency (RF) signals, propagating in typical environments, experience time dispersion. This time dispersion is known as multi-path and is caused by the RF signal being deflected off of various environmental surroundings such as buildings, mountains, moving objects, etc. Such a multi-path signal when received is corrupted by the addition of multiple replicas of the true signal. These replicas have differing amplitude, phase, and time delay with respect to the desired signal. In the receiver, the replicas must be accounted for and compensated for in order to achieve a quality demodulation of the signal. One such corrective measure is to equalize the multi-path signal to essentially achieve such a compensation.
Receivers incorporating equalization techniques to reduce the effects of multi-path are well known in the art. One such receiver is described by G. Ungerboeck, "Adaptive Maximum-Likelihood Receiver For Carrier-Modulated Data-Transmission Systems," IEEE Transactions on Communications, Vol. Com-22, pp. 624-635, May 1974. Basically, the received signal, possible corrupted by inter-symbol interference (ISI) due to the multi-path channel, other signals transmitted on the same RF carrier frequency from other parts of the system, known as co-channel interference, and additive noise due to the receiver front end, is equalized through the use of a maximum likelihood sequence estimator (MLSE) equalizer. The MLSE equalizer employs a complex matched fiber (CMF) which is matched to the impulse response of the multi-path channel, and a modified Viterbi algorithm (VA) section, per FIG. 2 in Ungerboeck.
Due to the rapid rate of change of the channel impulse response, coefficients used to construct the complex matched filter, which depend upon an estimate of the channel impulse response, must be generated frequently enough so that the coefficients accurately represent the channel during equalization. These coefficients are typically derived by correlating a predetermined synchronization pattern stored in the receiver with a synchronization pattern modulated with the received signal, the correlation being an estimate of the channel impulse response (there are other techniques, however, which obtain an estimate of the channel impulse response without using an explicit synchronization pattern). The combination of correlating and matched filtering (if the complex matched filter were the correct one) provides the function of removing phase offset between the incoming signal and the receiver's local oscillator and maximizing the signal-to-noise ratio of the received signal. Output from the complex matched filter is passed to the MLSE which accounts for the ISI problem stated earlier. The calculations performed by the MLSE to account for the ISI rely heavily on the channel impulse response estimate and also the complex matched filter derived from the estimated channel impulse response.
In the Group Special Mobile (GSM) Pan European Digital Cellular System, the synchronization sequence used to determine the correlation is relatively short therefore, the complex matched filter coefficients which are produced therefrom are highly susceptible to additive noise, interference, and cross-correlation products. When the correlation is noisy, not only is the matched filtering process adversely affected because the complex matched filter is not properly matched to the channel, but the MLSE processing is also adversely affected as well for the very same reason.
Thus, a need exists for a receiver which improves detection of a corrupted signal using equalization by improving the estimated channel impulse response which is used to generate coefficients for the complex matched filter and also is used by the MLSE.